1. Field of the Invention
The present invention relates to the fields of chemistry and pharmaceuticals. Embodiments of the present invention provide transition metal complexes of amino acids. Transition metal complexes of embodiments of the invention may be used as antimicrobial, anti-malarial, and anti-cancer agents, as well as catalysts in chemical reactions. Such compounds of the invention are particularly useful for combating multi-drug resistance against a broad range of microbials, including gram positive and gram negative bacteria.
2. Description of Related Art
Recent studies show there are currently too few drugs in the pipeline that offer improved treatment over existing drugs and which are capable of treating infections caused by ESKAPE pathogens. The ESKAPE pathogens, i.e., the species Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter, are known for causing a majority of hospital infections and for being able to escape the effectiveness of currently approved drugs. Multi-drug resistant strains such as MRSA are growing and the WHO estimates two billion people are infected with a latent form of M. tuberculosis. See Schwartz M., Aug. 9, 2006, Drug-resistant strains of tuberculosis are more virulent than experts assumed, Stanford Report. To address this issue, the Infectious Diseases Society of America has launched a collaborative effort called the 10×′20 initiative with the goal of introducing ten new, safe, and effective antibiotics by the year 2020. See Clin Infect Dis. (2010) 50 (8):1081-1083, e-published Mar. 9, 2010, doi: 10.1086/652237.
Metals are known to play important roles in biological systems. Generally this is limited to first row transition metals. Second and third row metals are generally thought of as toxic in nature. The use of transition metal compounds in medicine has a rich history with one of the most well-known compounds being cis-Platin, a platinum containing compound that is a very effective anti-cancer compound for hard tumors.
A review of the literature examining studies on the biological activities of transition metal complexes shows that the bulk of the studies have been directed toward finding compounds active against various types of cancers. See Dabrowiak, J. C., Metals in Medicine. First ed.; John Wiley & Sons, Ltd: West Sussex, UK, 2009; (b) Hansen, H. R.; Farver, O. In Metals in medicine: inorganic medicinal chemistry, CRC Press: 2010; pp 151-171; (c) Bruijnincx, P. C. A.; Sadler, P. J., New trends for metal complexes with anticancer activity. Curr. Opin. Chem. Biol. 2008, 12, 197-206. Other transition metal complexes traditionally used to treat cancer are listed below, as Formulas A-C:

Known mechanisms of action for transition metal drugs include having the structure for interstrand crosslink inhibiting (e.g., Formula D below) and preventing normal enzymatic functions of the DNA replication cycle. DNA has been isolated and examined using NMR and X-ray crystallographic studies performed. See Dabrowiak, Metals in Medicine; J. Wiley and Sons LTD, 2009 (Vol. 1).

Amino acids, the naturally occurring building blocks of proteins, make excellent ligands for transition metals, being able to bind in a bidentate fashion through the oxygen and nitrogen to the metal as illustrated below in Formulas E and F:

Traditionally, amino acid complexes and transition metals have been used in non-biological roles. Catalysis using amino acid derivatives is popular due to the modular nature of amino acids, (changing the R group for example), and that amino acids offer an inexpensive source of chiral building blocks. Amino acids and their derivatives are commonly used in the asymmetric reduction of ketones to their corresponding alcohols. See Manville, C. V.; Docherty, G.; Padda, R.; Wills, M., Application of Proline-Functionalised 1,2-Diphenylethane-1,2-diamine (DPEN) in Asymmetric Transfer Hydrogenation of Ketones, European Journal of Organic Chemistry 2011, (34), 6893-6901; See Carmona, D.; Viguri, F.; Pilar Lamata, M.; Ferrer, J.; Bardaji, E.; Lahoz, F. J.; Garcia-Orduna, P.; Oro, L. A., Ruthenium amino carboxylate complexes as asymmetric hydrogen transfer catalysts, Dalton Transactions 2012, 41 (34), 10298-10308; see Ahlford, K.; Adolfsson, H., Amino acid derived amides and hydroxamic acids as ligands for asymmetric transfer hydrogenation in aqueous media, Catalysis Communications 2011, 12 (12), 1118-1121; and see Breuil, P.-A. R.; Reek, J. N. H., Amino Acid Based Phosphoramidite Ligands for the Rhodium-Catalyzed Asymmetric Hydrogenation, European Journal of Organic Chemistry 2009, (35), 6225-6230 (“Breuil 2009”). They have also been used in asymmetric reduction of alkenes. See Breuil 2009.
The discovery of cis-platin and other related platinum complexes jump-started the investigation of the platinum group's potential biological role. See Rosenberg, B., Platinum compounds: a new class of potent antitumour agents, Nature (London) 1969, 222 (5191), 385-6. Due to this, a large variety of platinum based amino acid complexes have been created. See Chandrasekharan, M., Cysteine complexes of palladium(II) and platinum(II), Inorganica chimica acta 1973, 7 (1), 88-90; and see Vicol, O., Some complex combinations of Pd(II) with methionine, Journal of inorganic & nuclear chemistry 1979, 41 (3), 309-315; and see Ziegler, C. J.; Sandman, K. E.; Liang, C. H.; Lippard, S. J., Toxicity of platinum(II) amino acid (N,O) complexes parallels their binding to DNA as measured in a new solid phase assay involving a fluorescent HMG1 protein construct readout, JBIC, J. Biol. Inorg. Chem. 1999, 4; 402-411; and see Slyudkin, O. P.; Tulupov, A. A., Chiral complexes of Pt with amino acids: Synthesis, structure, properties, Russ. J. Coord. Chem. 2005, 31, 77-85.
The most extensive work on platinum group metals and amino acids was done by Wolfgang Beck and co-workers. in a series of articles titled “Metal Complexes with Biologically Important Ligands.” Their group has published on a variety of compounds with what are termed biologically important ligands and is at least up to 175 in a series of papers with this title, many of them being ligands derived from amino acids. See Schreiner, B.; Wagner-Schuh, B.; Beck, W., Metal complexes of biologically important ligands, CLXXV, Pentamethylcyclopentadienyl half-sandwich complexes of rhodium(III) and iridium(III) with Schiff bases from 2-(diphenylphosphino)benzaldehyde and alpha-amino acid esters, Zeitschrift fuer Naturforschung, B: A Journal of Chemical Sciences 2010, 65 (6), 679-686. The papers focused on synthesis, characterization, and interesting structural findings, but did not lend themselves to direct application.
Ruthenium based amino acid complexes have also been studied for their potential anti-cancer role. See Habtemariam, A.; Melchart, M.; Fernandez, R.; Parsons, S.; Oswald, I. D. H.; Parkin, A.; Fabbiani, F. P. A.; Davidson, J. E.; Dawson, A.; Aird, R. E.; Jodrell, D. I.; Sadler, P. J., Structure-Activity Relationships for Cytotoxic Ruthenium(II) Arene Complexes Containing N,N-, N,O-, and O,O-Chelating Ligands, J. Med. Chem. 2006, 49, 6858-6868. Other noble metals such as iridium and rhodium are often overlooked. Extension of amino acid based platinum metal systems to areas other than anti-cancer treatments is an area of interest as well.
It has been known to use organometallic compounds for their antimicrobial properties. Traditionally, synthesis processes focused on creating compounds similar to cis-platin. One such synthesis scheme is illustrated below in Scheme A. See, e.g., Vasić, G. P.; Glodjović, V. V.; Radojeviće, I. D.; Stefanović, O. D.; Comić, L. R.; Djinović, V. M.; Trifunović, S. R., Stereospecific ligand and their complexes: V. Synthesis, characterization and antimicrobial activity of palladium(II) complexes with some alkyl esters of (S,S)-ethylenediamine-N,N-di-2-propanoic acid, Inorg. Chim. Acta, 63 (2010) 3606-3610; ISSN: 0020-1693; DOI: 10.1016/j.ica.2010.05.046. Such compounds, however, routinely were found to have high cytotoxicity effects. Additionally, using these models, minimum inhibitory concentrations (MIC) achieved have only been around 30 ug/mL. Accordingly, due to the high toxicity and low effectiveness, work on these types of compounds has slowed.

The work was performed to provide a set of ligands that could have ester variation as well as stereo isomer variation using ethylene diamine derivatives, with palladium as the main metal of focus. Examples of such compounds are illustrated in Formulas G-I below.

Other attempts at synthesizing organometallic compounds for use as antimicrobials included using existing antimicrobials as ligands. In some cases, such compounds were shown to combat developed resistance in some organisms. In particular, the coordination of zinc, cadmium, nickel, palladium or platinum with such ligands has been studied. See Zengin, H.; Dolaz, M.; Golcu, A. Curr. Anal. Chem. 2009, 5, 358. Coordination showed to have a greater effect on inhibition than the antimicrobial by itself. An example of using an existing antimicrobial (Lorcarbef, or LOR) as a ligand is illustrated in Formula J.

Another approach involved the use of macrocyclic ligands. See Soni Rani, Sumit Kumar, Sulekh Chandra, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy Volume 78, Issue 5 2011, 1507-1514. The idea was to create a cyclic tetradentate ligand for Pd, Pt, Ru, and Ir and modify the —R groups on outer ring carbons to increase either hydrophobic or hydrophilic properties, as shown in Formula K.

Other techniques involving modifying current antimicrobials with an organometallic group to create new effects to overcome resistance have been used. For example, it has been known to modify pentamidine with Ir[COD]Cl2, which has been used to combat P. jirovecii, a severe type of pneumonia seen in HIV patients.

Most work for clinical applications have been with malaria (Plasmodium). For example, as illustrated in Formula M below, the incorporation and modification of chloroquine to metal arene structure is known to provide synergistic effects (Ferroquine—currently stage II). See Beckford, F.; Dourth, D.; Shaloski, M.; Didion, J.; Thessing, J.; Woods, J.; Crowell, V.; Gerasimchuk, N.; Gonzalez-Sarrias, A.; Seeram, N. P., Half-sandwich ruthenium-arene complexes with thiosemicarbazones: synthesis and biological evaluation of [(η6-p-cymene)Ru(piperonal thiosemicarbazones)Cl]Cl complexes, 2011 August; 105(8):1019-29, J. Inorg. Biochem. 2011. Still few compounds are available to combat other bacterial infections.

Very few studies, however, have examined the biological activity in general and anti-microbial activity in specific of amino acid complexes of transition metals. Al-Fregi et al researched the antibacterial activity of four complexes of the type [Pt(AA)(BAMC)] where AA is a dicarboxylate amino acid of glutamate or aspartate and the (BAMC) is a 1,4-bis(amino methylene)cyclohexane. See Al-Fregi, A. A.; Abood, H. A. A.; Al-Saimary, I. E., The antibacterial activity of 1.4 (amino methylene)cyclohexane platinum (II) and palladium (II) dicarboxylate amino acid complexes. Internet J. Microbiol. 2007, 4, DOI: 10.5580/1a1f. These complexes were studied in vitro against eight bacteria including Staph aureus, Staph epidermis, β-hemolytic streptococci, viridance streptococci, E. coli, Enterobacter, Klebsiella, and Pseudomonas aeruginosa. The lowest minimum inhibitory concentration (MIC) value found was 100 ug/mL, with most showing antibacterial activity only at 250 ug/mL or higher.
Spera et al evaluated palladium (II) complexes of S-allyl-L-cysteine through antibacterial assays. See Spera, M. B. M.; Quintao, F. A.; Ferraresi, D. K. D.; Lustri, W. R.; Magalhaes, A.; Formiga, A. L. B.; Corbi, P. P., Palladium(II) complex with S-allyl-L-cysteine: new solid-state NMR spectroscopic measurements, molecular modeling and antibacterial assays. Spectrochim Acta A Mol Biomol Spectrosc 2011, 78 (Copyright (C) 2011 U.S. National Library of Medicine.), 313-8. While their methodology does not allow for an accurate calculation of MIC values, ballpark calculations would indicate that the lowest possible MIC values are in the 200+ ug/mL range and probably significantly higher. The complex was most effective against Staph aureus (Gram positive), E. coli, and Pseudomonas aeruginosa (Gram negative). However, testing of simple palladium chloride complexes shows the activity most likely stems from the Pd+2 ion and not from any properties of the complexes themselves.
Currently there is limited research in the field of organometallic anti-microbials, a class of anti-microbials with promising potential. As can been seen from previous attempts at developing effective anti-microbials, what is especially needed are biologically active organometallic compounds to combat multi-drug resistant strains of bacteria.